Crack Development in a Low-Stress PBGA Package due to Continuous Recrystallization Leading to Formation of Orientations with [001] Parallel to the Interface

Abstract

Thermal cycling was imposed on plastic ball grid array (PBGA) packages with a small die, a package design that does not impose a large strain on solder joints. Less cracking was observed after 2500 cycles from 0°C to 100°C (with 10 min dwell times and 10 min ramps) than in a prior study with a higherstress package design, so these samples were thermally cycled (TC) to 6400 cycles to investigate the relationship between cracks, microstructure, and grain crystal orientation. Cracked joint locations within the package were identified using the dye and pry method, indicating that cracks were most often found in joints near the perimeter of the die. Using orientation imaging microscopy (OIM), cracks were observed in many joints having a variety of dominant crystal orientations where the c-axis was between 0°and $50°from the package interface. Continuous recrystallization processes occurred and caused gradual rotations of initial orientations that reduced the angle between the c-axis and the package interface. While cracks were observed in joints with a variety of orientations, cracks were highly correlated with recrystallized grains having the [001] c-axis nearly parallel to the interface (''red'' orientations) in those joints that did not initially have this orientation.

The Failure Mechanism of Recrystallization – Assisted Cracking of Solder Interconnections

“…The results showed that the evolution of microstructures in the strain concentration regions commenced with gradual evolution of the cellular solidification structure, but after some time, i.e., the incubation period, the microstructures changed discontinuously by recrystallization. A similar observation has been reported in [66].…”

A Mechanistic Thermal Fatigue Model for SnAgCu Solder Joints

“…However, nucleation of a new grain is only stable if the misorientations are large enough, so high-angle grain boundaries must occur abruptly. 26 In contrast, continuous recrystallization is a recoverydominated process leading to ongoing increases in boundary misorientation and conversion of lowangle boundaries into high-angle ones by coalescence and rotation.…”

“…27 This would tend to favor continuous, rather than discontinuous, recrystallization. 26 Also, the misorientation of the Ag 3 Sn precipitates is by nature high. Indeed, detailed studies have shown thermal cycling of SnAgCu joints to first lead to the formation of new subgrains within the individual b-Sn dendrites 12 ), and eventually between the densely spaced precipitates as well, after which these subgrains then continue to rotate during ongoing cycling.…”

“…Dynamic recrystallization occurs simultaneously with the creation of dislocations; by an ongoing coalescence and rotation of the dislocation cells, the recovery is accelerated by the excess vacancies. 26 However, this excess tends to disappear within seconds after the loading stops. 26 It is actually not easy to cause extensive recrystallization within a typical SnAgCu solder joint.…”

“…26 However, this excess tends to disappear within seconds after the loading stops. 26 It is actually not easy to cause extensive recrystallization within a typical SnAgCu solder joint. Sn has a very large number of different slip systems so it is not surprising 28 that it tends to form three-dimensional dislocation cell structures during inelastic deformation.…”